ISS Solar Alpha Rotary Joint Tribological Anomaly by Carlos Enriquez, January 2024

ISS Solar Alpha Rotary Joint Tribological Anomaly

Carlos Enriquez

NASA

November 28, 2023

Abstract:

In early September 2007, 2 months after activation on theInternational Space Station (ISS), the starboard Solar Alpha Rotary Joint(SARJ) reported that drive command current started increasing from a nominalpeak value of approximately 0.2 amps to a peak value of approximately 1.2 ampsin a period of 6 weeks. As the current increased, onboard accelerometermeasurements and visual observation of increased vibrations on the ISSstructure were a concern. Shortly afterthese increases in current and vibration, the decision was made to stop thestarboard SARJ from autotracking and avoid further rotations until the cause ofthe current increase could be determined. Operation of the port side SARJ wasnominal and was not interrupted.

Given the on-orbit observations, review of buildand test records, tests, analyses, and simulations performed, a most probable tribologicalSARJ anomaly root cause was established and is presented here. The kinematicsof the Trundle Bearing Assembly (TBA) and Drive Lock Assembly (DLA) mechanismsrequire that the roller thrust loads (related to friction coefficient andmistracking angle) be controlled to ensure stable roller line contact with therace ring surfaces. Inadequate lubrication of the roller/race ring interfacecombined with roller mistracking angles within specification resulted in thrustloads, high enough, to cause at least some of the TBA or DLA rollers to edgeload as the SARJ rotated. The rollerprofile geometry was not adequately controlled during the design phase. When a roller is edge loaded, the preload onthat roller is concentrated on a reduced contact area resulting in high contactstresses and shear stresses in the race ring case and core. These stresses exceeded the allowable bearingstrength capability of the race ring case and core, leading to brittle fractureand spalling of the nitrided layer from the starboard SARJ race ring.

Biography:

Carlos Enriquez obtained a B.S. Degree in Aerospace Engineering from the University of Arizona in 1989, and a M.S. Degree in Technical Management from Embry Riddle University in 2001. Upon graduating from the University of Arizona, Mr. Enriquez was hired by The Boeing Company to join the teams developing the International Space Station, initially with the Guidance, Navigation and Control Team, later joining the Structures and Mechanisms Team. During the development of the ISS, he participated in the development of various mechanical attachment systems, which performed on-orbit assembly operations by robotic berthing, EVA and docking to the ISS. He also participated in the development of rotary mechanical systems, such as the Solar Alpha, and the Thermal Radiator Rotary Joints. Before transitioning to NASA in 2021, he led the AITs for the NASA Docking System Block 1 (NDSB1) and the International Space Station Rollout Solar Arrays (iROSA), from inception to hardware delivery. Currently, Mr. Enriquez is the NASA Commercial Crew Structures and Mechanisms Manager.